The United States Department of Defense created the Global Positioning System (GPS) to allow military ships, aircraft and ground vehicles to determine their location anywhere in the world. GPS consists of a satellite segment, a ground control segment, and user receivers. The satellite segment consists of 24 satellites placed asymmetrically in six orbital planes where each plane is inclined by 55.degree. relative to the equatorial plane. Each satellite continuously broadcasts direct-sequence, spread-spectrum signals on which passive receivers can perform precise ranging measurements. Each broadcast is also modulated with a navigation data message, which is developed by the ground control station and provides data to the user such as the satellite location and clock error. For each of several satellites, the user equipment measures a "pseudo range" and demodulates the navigation message. The pseudo range is equal to the true range from the receiver to the satellite plus errors, either intentionally or unintentionally, introduced in the range measurement. Pseudo range measurements to four well-spaced satellites are sufficient to solve for the user's three-dimensional position and clock offset.
GPS includes the standard positioning service (SPS) which provides civilian users with 100 meter accuracy. Civilian users are guaranteed access to a 1.023 MHz spreading code (the C/A code) which modulates a signal at a frequency of 1575.42 MHz. Errors are introduced, however, due to several slowly varying biases arising from ionospheric refraction, tropospheric refraction, stability of the satellite clock, predictability of the orbit of the satellite, stability of the receiver, direct sequence tracking, and selective availability. Selective availability is the largest error source and is intentionally introduced by the Department of Defense for national security reasons. With selective availability enabled, the SPS provides 100 meter horizontal accuracy. Additional information regarding GPS can be found in "The Global Positioning System: Signals, Measurements, and Performance" by Per K. Enge, INTERNATIONAL JOURNAL OF WIRELESS INFORMATION NETWORKS, Volume 1, No. 2, 1994.
Another approach to providing precision navigation involves the use of the Wide Area Augmentation System (WAAS) utilized by the Federal Aviation Agency (FAA) and available to the general public. The WAAS system improves the integrity, accuracy, availability and continuity of the GPS Standard Positioning Service (SPS). The WAAS system includes a terrestrial network of stations (wide area reference stations) monitoring the performance of GPS satellites, and a WAAS geostationary satellite. The reference stations continuously report to regionalist master stations (wide area master stations). The master stations process the observed data to determine SPS signal corrections and whether a system performance fault has occurred. The SPS signal corrections and a system performance fault, if any, is up-linked to a geostationary satellite via a command antenna (ground earth station) for immediate rebroadcast by the satellite to users using the WAAS service.
The WAAS system provides correction data to all users that have a WAAS receiver. The WAAS system, however, fails to provide a user with precise location information since all users throughout the Continental United States receive the same SPS correction signals. Furthermore, this system does not have a way to create an accounting record so the user can be subsequently charged for receiving the correction data. In addition, there is a delay in providing the user with signal integrity since a system fault has to first be determined by the master stations and then transmitted to the geostationary satellite before it is broadcasted to the user.